<?php
/**
* Google Authenticator 
* google 动态密钥算法生成密钥
*公用密钥
*$InitalizationKey = "PEHMPSDNLXIOG65U";                
* 时间戳
*$TimeStamp    = Google2FA::get_timestamp();
* 生成secretkey
*$secretkey    = Google2FA::base32_decode($InitalizationKey);    
* 生成密码
*$otp          = Google2FA::oath_hotp($secretkey, $TimeStamp); 
* 验证
* $result = Google2FA::verify_key($InitalizationKey, "123456");
*/
class Google2FA {

    const keyRegeneration   = 30;   // Interval between key regeneration
    const otpLength     = 6;    // Length of the Token generated

    private static $lut = array(    // Lookup needed for Base32 encoding
        "A" => 0,   "B" => 1,
        "C" => 2,   "D" => 3,
        "E" => 4,   "F" => 5,
        "G" => 6,   "H" => 7,
        "I" => 8,   "J" => 9,
        "K" => 10,  "L" => 11,
        "M" => 12,  "N" => 13,
        "O" => 14,  "P" => 15,
        "Q" => 16,  "R" => 17,
        "S" => 18,  "T" => 19,
        "U" => 20,  "V" => 21,
        "W" => 22,  "X" => 23,
        "Y" => 24,  "Z" => 25,
        "2" => 26,  "3" => 27,
        "4" => 28,  "5" => 29,
        "6" => 30,  "7" => 31
    );

    /**
     * Generates a 16 digit secret key in base32 format
     * @return string
     **/
    public static function generate_secret_key($length = 16) {
        $b32    = "234567QWERTYUIOPASDFGHJKLZXCVBNM";
        $s  = "";

        for ($i = 0; $i < $length; $i++)
            $s .= $b32[rand(0,31)];

        return $s;
    }

    /**
     * Returns the current Unix Timestamp devided by the keyRegeneration
     * period.
     * @return integer
     **/
    public static function get_timestamp() {
        return floor(microtime(true)/self::keyRegeneration);
    }

    /**
     * Decodes a base32 string into a binary string.
     **/
    public static function base32_decode($b32) {

        $b32    = strtoupper($b32);

        if (!preg_match('/^[ABCDEFGHIJKLMNOPQRSTUVWXYZ234567]+$/', $b32, $match))
            throw new Exception('Invalid characters in the base32 string.');

        $l  = strlen($b32);
        $n  = 0;
        $j  = 0;
        $binary = "";

        for ($i = 0; $i < $l; $i++) {

            $n = $n << 5;               // Move buffer left by 5 to make room
            $n = $n + self::$lut[$b32[$i]];     // Add value into buffer
            $j = $j + 5;                // Keep track of number of bits in buffer

            if ($j >= 8) {
                $j = $j - 8;
                $binary .= chr(($n & (0xFF << $j)) >> $j);
            }
        }

        return $binary;
    }

    /**
     * Takes the secret key and the timestamp and returns the one time
     * password.
     *
     * @param binary $key - Secret key in binary form.
     * @param integer $counter - Timestamp as returned by get_timestamp.
     * @return string
     **/
    public static function oath_hotp($key, $counter)
    {
        if (strlen($key) < 8)
        throw new Exception('Secret key is too short. Must be at least 16 base 32 characters');

        $bin_counter = pack('N*', 0) . pack('N*', $counter);        // Counter must be 64-bit int
        $hash    = hash_hmac ('sha1', $bin_counter, $key, true);

        return str_pad(self::oath_truncate($hash), self::otpLength, '0', STR_PAD_LEFT);
    }

    /**
     * Verifys a user inputted key against the current timestamp. Checks $window
     * keys either side of the timestamp.
     *
     * @param string $b32seed
     * @param string $key - User specified key
     * @param integer $window
     * @param boolean $useTimeStamp
     * @return boolean
     **/
    public static function verify_key($b32seed, $key, $window = 4, $useTimeStamp = true) {

        $timeStamp = self::get_timestamp();

        if ($useTimeStamp !== true) $timeStamp = (int)$useTimeStamp;

        $binarySeed = self::base32_decode($b32seed);

        for ($ts = $timeStamp - $window; $ts <= $timeStamp + $window; $ts++)
            if (self::oath_hotp($binarySeed, $ts) == $key)
                return true;

        return false;

    }

    /**
     * Extracts the OTP from the SHA1 hash.
     * @param binary $hash
     * @return integer
     **/
    public static function oath_truncate($hash)
    {
        $offset = ord($hash[19]) & 0xf;

        return (
            ((ord($hash[$offset+0]) & 0x7f) << 24 ) |
            ((ord($hash[$offset+1]) & 0xff) << 16 ) |
            ((ord($hash[$offset+2]) & 0xff) << 8 ) |
            (ord($hash[$offset+3]) & 0xff)
        ) % pow(10, self::otpLength);
    }
}
